The present invention is directed to an optical unit having at least one longitudinal side coupling zone. The optical unit utilizes at least one glass fiber which has a core surrounded by a glass cladding layer with a step in the index of refraction from a higher to a lower value occurring at the junction between the core and the cladding layer.
Glass fibers having a core surrounded by a glass cladding layer are disclosed in an article by T. Ozeki and B. S. Kawasaki, "Optical Directional Coupler Using Tapered Sections In Multimode Fibers," Applied Physics Letter, Vol. 28, No. 9, May 1 1976, pp. 528, 529. In this article, a longitudinal side coupling zone is formed by local constriction of the overall light conductor in which the core diameter and the casing diameter are consequently reduced in comparison to the diameter of the remaining portion of the fiber. Light conductors of this kind are used for mixers wherein the coupling zone of a plurality of light conductors are coalesced and it is endeavored to achieve as dense as possible packing. However, an increasing number of fibers impedes a dense packing because the difference between the diameter of the group in the coupling zone and outside the zone rapidly increases and subsequently the outer fibers are subjected to considerable deflection in the coupling zone. The bending stability and the target stability of individual fibers are subject to limits and these are particularly true in the zone of the constriction.